Merge g3 into g4

# Conflicts:
#	ROADMAP.md
#	crates/engine/src/cmd/edit.rs
#	crates/engine/src/cmd/formulas.rs
#	crates/engine/src/cmd/inspect.rs
#	crates/engine/src/cmd/pivot.rs
#	crates/ui-egui/src/lib.rs
#	crates/ui-egui/src/tabs.rs
#	docs/gaps.md
#	docs/ui-parity.md

crates/engine/src/cmd/mod.rs

@@ -19,6 +19,7 @@ pub mod sheet;
 pub mod slicer;
 pub mod solver;
 pub mod spelling;
+pub mod stash;
 pub mod view;
 pub mod whatif;
 
@@ -96,12 +97,19 @@ pub fn has_doc(s: &Session) -> std::result::Result<(), String> {
 }
 pub fn can_undo(s: &Session) -> std::result::Result<(), String> {
     has_doc(s)?;
+    not_grouping(s)?;
     if s.active().is_some_and(|d| !d.undo.is_empty()) { Ok(()) } else { Err("nothing to undo".into()) }
 }
 pub fn can_redo(s: &Session) -> std::result::Result<(), String> {
     has_doc(s)?;
+    not_grouping(s)?;
     if s.active().is_some_and(|d| !d.redo.is_empty()) { Ok(()) } else { Err("nothing to redo".into()) }
 }
+/// Undo and Redo wait while an undo group is open on the active document (an action that spans
+/// several calls is still running).
+fn not_grouping(s: &Session) -> std::result::Result<(), String> {
+    if s.active().is_some_and(|d| s.in_group(d.uid)) { Err("finish or stop the running action first (automate.stopAction)".into()) } else { Ok(()) }
+}
 pub fn has_clipboard(s: &Session) -> std::result::Result<(), String> {
     has_doc(s)?;
     if s.clipboard.is_some() { Ok(()) } else { Err("the clipboard is empty".into()) }
@@ -157,7 +165,7 @@ pub fn find_command(id: &str) -> Option<&'static CommandSpec> {
     test_commands().iter().find(|c| c.id == id).copied()
 }
 
-static TEST_COMMANDS: std::sync::Mutex<Vec<&'static CommandSpec>> = std::sync::Mutex::new(Vec::new());
+static TEST_COMMANDS: std::sync::Mutex<Vec<&CommandSpec>> = std::sync::Mutex::new(Vec::new());
 
 /// Test-only: register an extra command so UI integration tests can prove a pending
 /// keytip becomes live the moment its command lands (see crates/ui-egui/tests/keytips.rs).
@@ -179,6 +187,23 @@ pub fn register_test_command(id: &'static str, run: Run) {
     test_commands().push(spec);
 }
 
+/// Test-only: an undoable command, to exercise how `execute` records its step.
+#[cfg(test)]
+pub(crate) fn register_undoable_test_command(id: &'static str, run: Run) {
+    let spec: &'static CommandSpec = Box::leak(Box::new(CommandSpec {
+        id,
+        label: id,
+        menu: &[],
+        shortcut: None,
+        params: "",
+        enabled: |_| Ok(()),
+        run,
+        journal: false,
+        undoable: true,
+    }));
+    test_commands().push(spec);
+}
+
 fn test_commands() -> std::sync::MutexGuard<'static, Vec<&'static CommandSpec>> {
     #[allow(clippy::unwrap_used)] // a poisoned test registry has no meaningful recovery
     match TEST_COMMANDS.lock() {
@@ -527,9 +552,15 @@ pub fn auto_row_height(wb: &mut Workbook, si: usize, row: u32) {
         return;
     }
     let mut need: f32 = default;
+    // The cell loop must not rescan every merge per cell: the merges touching this row are
+    // picked out once, and per cell the candidates are re-checked (normally none or a couple;
+    // a sheet with many merges and many saved columns would otherwise scan all of them per
+    // cell, rows × merges × columns).
+    let row_merges: Vec<gridcraft_core::RangeRef> =
+        sh.merges.iter().copied().filter(|m| !m.is_single() && m.start.row <= row && row <= m.end.row).collect();
     for (c, cell) in sh.cells.row(row, 0, gridcraft_core::MAX_COLS - 1) {
         // Merged cells never auto-fit their rows, like Excel (column autofit skips them too).
-        if sh.merge_at(CellRef::new(row, c)).is_some_and(|m| !m.is_single()) {
+        if row_merges.iter().any(|m| m.contains(CellRef::new(row, c))) {
             continue;
         }
         let st = wb.styles.get(cell.style);